This invention relates to continuous casting systems and more particularly to the removal of foreign matters, such as asbestos and waste materials produced in manufacturing castings, accumulated between casting rolls of the continuous casting system, and to countermeasures against various troubles caused by magnetic substances included in the foreign matters.
In a continuous casting system for casting metal such as for instance steel, a number of rolls are arranged in the direction of drawing castings and the distances between these rolls are very short, and yet a large number of casting cooling spray pipes are provided close to one another between the rolls thus arranged.
In manufacturing castings by the continuous casting system thus organized, frequently considerable segregation occurs and pores and prismatic crystals are produced in the castings. In order to solve these difficulties, a technique of electromagnetically agitating the molten metal left in a casting for its certain length measured from the mold has been proposed. In this method, an electromagnetic agitator is positioned between predetermined rolls.
When in the continuous casting system described above, a casting is drawn out of the mold, asbestos, shavings on a dummy bar and other waste materials created in cooling the casting are dropped and accumulated under the rolls in the vicinity of the electromagnetic agitator, and in addition scales peeled off the casting are dropped; that is, a large quantity of foreign matters is deposited under the rolls near the electromagnetic agitation during the drawing operation of casting. This is one of the difficulties accompanying the conventional continuous casting system.
More specifically, the foreign matters thus dropped and accumulated often cause casting cooling nozzles to be clogged up although the casting cooling process plays an important part in manufacturing castings, that is, they affects a casting drawing speed and a casting quality. Furthermore, these foreign matters cause the rolls to irregularly rotate or stop at worst, as a result of which the rolls are worn at only one side thereby reducing their service lives, and the surface of a casting is scratched.
Therefore, in the case when such foreign matters are deposited between the rolls, it is necessary to suspend the operation of the casting system so that a person can go in the narrow space between the rollers to remove the foreign matters. However, the environmental conditions of the continuous casting system are, in general, worse and hazardous when viewed in the light of safety. More specifically, the temperature in a room where the continuous casting system is operated is very high, and the electromagnetic agitator is provided with its electric system in the vicinity of the rollers and therefore the hazard of electric leak may be encountered in the removal of the foreign matter. Accordingly, it is impossible to smoothly and quickly achieve the removal of foreign matters. This is a waste of time in the drawing operation of casting.
In the case where the foreign matters are magnetic substances, the magnetic substance in the magnetic field of the electromagnetic agitator induces a forece of vibration whose frequency is twice as many as the frequency of the power supply of the agitator. This force of variation is much greater than the force of an induction current contributing to agitating the molten metal in a casting, and breaks the rolls. As a result, the shell of the casting is damaged, and at worst the casting is broken.
With the continuous casting system, there are a number of chances where magnetic substances have a bed influence on the operation thereof. Briefly, these chances occur in the following three cases (1), (2) and (3).
Case (1) where a so-called "dummy bar" including magnetic substances is employed when a pouring operation starts.
Case (2) where a drawing speed is excessively reduced while steel is poured into the mold or where the steel pouring operation is temporarily suspended, as a result of which the surface temperature of a casting in the electromagnetic agitator is lowered below the Curie temperature.
Case (3) where the scales of a steel material drop to the electromagnetic agitator while it being poured into the mold, or where magnetic ground metals are caused to drop the agitator by the leakage of molten metal.
In case (1), the force of vibration described above is certainly produced. Cases (2) and (3) are abnormal conditions in manufacturing casting, and in these cases the force of vibration is not always induced, however there are chances in which magnetic substances approach the electromagnetic agitator. Therefore, in all of the cases (1), (2) and (3), some countermeasure must be taken so that no force of vibration is caused by magnetic substances.
In case (1), the operator observing the movement of the dummy bar turns off the electromagnetic agitator before it reaches the agitator and turns on the agitator later when the dummy bar has moved out of the magnetic field thereof. In case (2), after the operator has detected that the drawing speed is extremely reduced or the pouring operation is suspended, he controls the operation of the agitator in the same manner as in case (1). In case (3) also, the operation of the agitator is controlled by the operator after he has detected the production of the vibration. Accordingly, the timing of turning on and off the electromagnetic agitator is liable to be unsuitable. This is an obstruction to the effective, smooth operation of the continuous casting system which has been improved high in processing speed and larging in processing capacity.